1. Field of the Invention
This invention relates generally to catalyst apparatus for reducing the emissions of internal combustion engines and more specifically to a heat-isolated catalytic reactor for reducing the emissions of diesel engines intended for use in underground mines and other similar or potentially inflammable or inadequately ventilated environments.
2. Background of the Invention
Diesel engines power a wide variety of vehicles and equipment used in various underground and mining applications due to their improved safety and efficiency over electrically powered vehicles and equipment. However, diesel engines are not without their disadvantages and there remain several problems that need to be solved before diesel engines can be fully utilized in such environments.
Perhaps the most significant problem that has limited the use of diesel engines in mines or other environments having limited ventilation, and with increasing environmental awareness, is becoming a concern in all diesel engine operations, is that the exhaust from the engines contains numerous components thought to be harmful to humans, such as unburned hydrocarbons, carbon monoxide (CO), oxides of nitrogen (NO.sub.x), sulfur dioxide (SO.sub.2), sulfates, and solid particulate matter. The solid particulate matter typically comprises small, solid, irregularly shaped particles, which are themselves agglomerates of smaller sub-particles. The particles may also have high molecular weight hydrocarbons absorbed on their surfaces. Frequently, the particulate matter is a complex mixture of pure carbon and various kinds of organic materials, and the sizes may range from very small particles of about 0.01 microns to relatively large clusters in the range of 10-30 microns, giving the particulate an extremely fine and light, flour-like consistency. Turbocharged diesel engines tend to emit more of the smaller particles with much lower levels of retained organic compounds. Particle sizes of 10 microns and less are considered to be the most damaging to human lungs, and certain characteristic components of diesel exhaust particulate emissions are known carcinogens.
Another problem which has limited the usefulness of diesel engines in such environments is that diesel engines may emit sparks or flames caused by backfiring through the intake and exhaust manifolds. Of course, any emitted sparks or flames may ignite the various ignitible dusts or explosive gases typically found in the air in underground mines. Also, since the internal operating temperatures of such engines may exceed 1200.degree. F., the external surfaces of the engines may be hot enough to trigger a fire or explosion if ignitable dusts accumulate on the hot external surfaces of the engine or if inflammable liquids come in contact with those hot surfaces.
Several devices have been developed and are being used in attempts to make diesel engines more suitable for use in environments having limited ventilation or containing explosive atmospheres. For example, many manufacturers have reduced the particulate emissions of diesel engines by limiting the amount of fuel injected under acceleration and high load (i.e., lug-down) conditions. However, reducing the amount of fuel injected during acceleration and lug-down operations is not effective to eliminate all solid particulate emission, or even decrease it to a desirably low level, unless the power output of the engine is reduced to an unacceptably low level.
Several alternative systems have also been developed in attempts to find a more effective means of reducing the solid particulate emissions. Principle among these alternative systems are catalysts for catalytically (i.e., thermally) oxidizing the particulate matter while it is still entrained in the exhaust gas, systems for thermally oxidizing filter-trapped particulate matter, and systems for catalytically oxidizing filter trapped particulate matter. Water scrubbers have also found fairly wide-spread applicability in the underground mining environment. Examples of such systems are disclosed in U.S. Pat. No. 3,771,967 issued to Nowak; U.S. Pat. No. 3,786,635 issued to Kates et al.; U.S. Pat. No. 3,886,738 issued to Sien; U.S. Pat. No. 3,903,694 issued to Aine; U.S. Pat. No. 4,075,994 issued to Mayer et al.; U.S. Pat. No. 4,133,654 issued to Hill et al.; U.S. No. 4,338,784 issued to Liu et al.; U.S. Pat. No. 4,345,429 issued to Yasuhara; U.S. Pat. No. 4,671,060 issued to Wilkens; and U.S. Pat. No. 4,864,821 issued to Hoch.
Unfortunately, none of these systems has proven to be a panacea, and there remain a number of serious shortcomings which have tended to make them unsuitable for use, particularly in the underground mining environment. For example, in-stream thermal oxidation techniques require the provision to the exhaust stream of large amounts of heat energy to oxidize the particulate matter, which heat is usually unrecoverable, thus reducing efficiency of the system. In-stream catalytic oxidation methods, such as those disclosed by Mayer et al. and Yasuhara do not require additional energy, but have problems of their own. For example, before a catalyst can be effective in oxidizing unburned hydrocarbons, carbon monoxide, and aerosols, it must be placed in the hot exhaust gases and allowed to reach temperatures sufficient to trigger the oxidization process, usually in the range of 400.degree. F.-500.degree. F. The heat released by the oxidation of the exhaust gases further heats the catalyst, so that most catalysts actually reach rather high steady state operating temperatures in the range of 1000.degree. F. to 1400.degree. F. Obviously, adding a catalyst to engines which must meet low surface temperature limits introduces at least two significant difficulties. First is the problem of insulating the catalyst so that the surface temperature of the catalyst housing does not exceed 300.degree. F. Second, the catalyst itself must be sufficiently insulated from the cooling jacket surrounding the catalyst housing so that the catalyst material can reach its normal operating temperature. While the patent issued to Nowak teaches the use of a catalytic converter having a hardened fibrous lining to resiliently support, insulate, and secure a monolithic catalyst element within an exhaust pipe, thus providing improved vibration resistance, Nowak's system does not provide sufficient heat insulation to allow the catalyst to work effectively within a water cooled exhaust pipe. In short, these problems have proven formidable, and catalyst systems have never enjoyed significant success on mine certified diesel engines or engines having water cooled exhaust systems.
Particulate emission has also been reduced by using a filter to trap the particulate matter before it escapes into the surrounding atmosphere. Ceramic materials, stainless steel wire mesh, and other filter materials capable of withstanding the high-temperature exhaust gases have been tried and are being used. The patents issued to Hoch and Yasuhara disclose variations on this theme. Unfortunately, because of the large quantities of particulate matter that are generated by most diesel engines, such filters clog quickly, which increases back pressure in the engine exhaust and affects the performance and efficiency of the engine. Of course, replacing the filter when the back pressure exceeds some predetermined limit would be helpful. However, the metal or ceramic materials used in most effective filters are expensive, so it is simply not practical to throw away such filters when they become clogged.
Several filter regeneration methods have been developed in attempts to make such particulate filter systems reusable. The most common filter regeneration methods rely on either thermal or catalytic oxidation of filter-trapped particulates. Unfortunately, the space, cost, and energy consumption required by such regeneration methods are substantial. Furthermore, in-situ filter regeneration techniques, where the filters rely on the hot exhaust gases themselves to raise the temperature of the filter high enough to oxidize the trapped particles, do not work with the light duty-cycles typically associated with underground engines. Consequently, the high temperature filters used on such engines must be removed and regenerated at some off-site location.
As mentioned above, water scrubbing systems have enjoyed a fair degree of success in reducing the exhaust emissions from engines used in underground mining environments. Most water scrubbing systems, such as those disclosed by Sien and Hill et al., comprise a water-filled baffle chamber that is connected to the exhaust manifold of the engine. The exhaust gases from the engine are bubbled through the water in the chamber, thus cooling the exhaust gases and removing a small percentage (about 10%) of the solid particulate matter. One positive aspect of water scrubbers is that they make excellent flame arresters, which, of course, has made them attractive for use in inflammable atmospheres, such as those typically associated with underground mines. Unfortunately, however, water scrubbers consume relatively large amounts of water and must be thoroughly cleaned at very frequent intervals. Another disadvantage is that water scrubbers do not remove carbon monoxide, oxides of nitrogen, or other gaseous pollutants from the exhaust gases and are only marginally effective in removing the water-soluble sulfur dioxide (SO.sub.2) from the exhaust gases. However, even the removal of the sulfur dioxide creates problems because the absorbed sulfur dioxide reacts with the water to form sulfuric acid (H.sub.2 SO.sub.4), which is eventually emitted from the exhaust system along with the exhaust gases. Furthermore, recent changes in the laws regulating the emissions of mine certified diesel engines have tightened the emission requirements to the point were most water scrubbers just cannot meet the new, more rigorous emission requirements.
The patent issued to Wilkens and assigned to the assignee of this invention uses a "dry" heat exchanger to cool the exhaust gases to avoid some of the problems associated with the water scrubbers. Unfortunately, however, Wilkens' heat exchanger tends to accumulate soot deposits quite rapidly, which significantly reduces the thermal transfer efficiency of the heat exchanger. Consequently, the Wilkens heat exchanger must be disassembled and thoroughly cleaned at frequent intervals; an expensive and time-consuming process. Moreover, the Wilkens' system cannot meet the new exhaust emission requirements because it does not remove any of the solid particulate matter or gaseous pollutants from the engine exhaust,
Some systems have been developed in which the relatively expensive high temperature filters are replaced with cheaper, preferably disposable, low temperature filters. Water scrubbers are used to cool the exhaust gases before they pass through the low temperature filters. However, the moist exhaust gas exiting the water scrubbers tends to foul and clog the low temperature filters quite rapidly, and, as mentioned above, water scrubbers have their own problems and maintenance costs.
Finally, Liu et al. teach an electrostatic particle collector to remove the particulate matter from the exhaust gases. However, the high voltages required by that kind of system introduce yet another explosion hazard when used in underground environments or other environments having inflammable atmospheres.
Consequently, there remains a need for an improved diesel engine emission reduction system that is suitable for use in underground mines or in other environments that have explosive or poorly ventilated atmospheres, or in environments where it is essential that the quantities of solid particulates in the diesel exhaust be kept to a minimum. Such a system must meet the rigorous requirements for spark and flame suppression and for maximum surface and exhaust temperature, while at the same time providing an economical and low maintenance method of removing the solid particulate matter, and preferably some of the carbon monoxide and other pollutants, from the exhaust gas. Ideally, such a system would include a catalytic reactor to oxidize unburned hydrocarbons, carbon monoxide and aerosols to increase the service life of a downstream particulate filter, yet be able to function effectively within a water cooled exhaust manifold and without increasing the surface temperature of the exhaust system to dangerous levels. Until this invention, no such catalytic reactor existed.